FIG. 15 is a partially cut away side view of an integrated hearing aid receiver disclosed in U.S. Pat. No. 5,193,116. FIG. 16 is an end view of interior elements of the integrated receiver of FIG. 15, with the housing and upper structure removed. The hearing aid receiver 210 comprises a housing 212 having first and second outlet ports 214, 216, respectively. A diaphragm 218 is disposed within the housing 212, defining an output chamber 220 and a motor chamber 222. An armature 224 is disposed within the motor chamber 222 and has an operative element comprising a fixed end 239a and a movable end 228. The armature 224 is coupled by a link 230 to drive the diaphragm 218. A permanent magnet structure 232 having a central passage 234 surrounds the movable end 228 of the armature 224 and provides a permanent magnetic field within the passage 234. A drive coil 236 is disposed about the armature 224 and is located proximate to the permanent magnet structure 232. An amplifier 238 is disposed within the motor chamber 222 and between the armature 224 and the diaphragm 218.
The housing 212 is generally rectangular in cross-section, having. generally planar top 212a, bottom 212b and side walls 212c. The a armature 224 is configured as a generally U-shaped strap having first and second opposed legs 239a, 239b, respectively. The first leg 239a is adhesively secured to the housing wall of the motor chamber 222 opposite the diaphragm 218 by means of adhesive 240.
The permanent magnet structure 232 comprises a stack of ferromagnetic laminations 242, each having an aligned central lamination aperture 244. A pair of permanent magnets 246, 248 are disposed within the lamination apertures 244 and cemented to opposite faces thereof. The lower faces of the laminations 242 are welded to the right most end of the fixed leg 239a of the armature 224. This serves to complete the magnetic circuit around the armature loop.
As will be noted from FIG. 16, the second leg 239b of the armature 224 is narrower than the first leg 239a. The second leg 239b terminates in the movable end 228 of the armature 224.
In operation, excitation of the drive coil 236 magnetizes the armature 224. Interaction of the armature movable end 228 with the magnetic field causes the armature movable end 228 to vibrate. Movement of the coupled diaphragm 218 produces sound in the output chamber 220, which passes to the outlet port 214 through a passage 250.
Other examples of transducers suitable for use in hearing aids are disclosed in U.S. Pat. Nos. 3,588,383, 4,272,654, and 5,193,116, which are incorporated by reference herein.
The sound pressure output of a receiver such as the receiver described above is created by the travel, or deflection, of an armature of the receiver when the armature vibrates. Maximum deflection of the moving armature creates maximum sound pressure output for a given armature geometry. The maximum deflection of an armature is limited by the magnetic saturation of the armature, which is governed by the maximum magnetic flux that can pass through the armature geometry. Therefore, one way to increase the sound pressure output is to increase the magnetic flux that can pass through the armature.
The magnetic flux is limited by material type and cross-sectional area of the armature. Thus, if the thickness of the armature is increased, the maximum magnetic flux that can pass through the armature geometry is increased. Increasing the thickness of the armature, however, also increases the stiffness of the armature and tends to reduce the maximum deflection of the armature. Thus, merely increasing the thickness of the armature does not provide a significant improvement in the maximum deflection of the armature.